The present invention relates to apparatus for transferring and heat-treating divided solids such as powder materials.
Apparatuses presently in use in industry for transfer and heat-treatment purposes comprise a transfer member and heat-treatment heater means.
In a first type of apparatus, the transfer member is a vibrating tubular case receiving the divided solids. The heat-treatment is then provided by heating the tubular case. Such apparatuses are difficult to implement and are relatively expensive.
A second type of apparatus is known in which the transfer member is a screw mounted to rotate about a longitudinal axis inside a tubular case. The heater means is constituted either by the case (which then constitutes a double wall for circulating a heat-conveying fluid), or by heating resistance elements fixed on the outside of the case, or by a tube around which the screw is wound, which tube conveys a fluid that is raised to a high temperature. Apparatuses of the above type are simpler to implement than the previously-mentioned apparatuses. Nevertheless, in the second type of apparatuses, only those solids which are close to the heating case or to the heating tube are subjected to an effective flow of heat. When a heating tube is used, it has also been found that since the tube does not perform any function associated with transferring or stirring the divided solids, the divided solids that are located in the vicinity of the tube are not renewed. As a result, the heating of the divided solids is incomplete, irregular, and not very effective. In order to heat particles located in positions that are remote from the tube, it becomes necessary to increase the temperature of the tube which runs the risk of turning the particles close to the tube into a crust because they are overheated, thereby disturbing flow and increasing maintenance costs (frequent cleaning is essential so as to remove particles sticking to the tube), or else the tubular case must also be heated, thereby further increasing the cost of the installation.
Finally, dual-case screws are known having an inner passage through which a heating fluid is passed. Nevertheless, that type of structure presents numerous drawbacks: in addition to the cost of high manufacturing costs and complex assembly for maintaining sealing while the screw is in rotation, thermal inertia becomes very high, thereby making it impossible to change temperature conditions quickly. This can be of great importance from a safety point of view in the event of a dangerous reaction occurring, in which case it is necessary to stop heating the substances being transferred as quickly as possible.
The object of the present invention is to remedy the above-specified drawbacks by designing apparatus which provides better performance.
According to the invention, this object is achieved by apparatus for transferring and heat-treating divided solids, the apparatus comprising at least one transfer member having a longitudinal axis and a helical portion mounted to rotate about its own longitudinal axis inside a tubular case and connected to the outlet shaft of a rotary drive motor, at least the helical portion of the transfer member being constituted in its bulk by an electrically conductive material, and the transfer member having connection means for connecting it to an electricity power supply so that it itself constitutes the heater means.
Thus, the heater means is constituted by the transfer member with which most of the divided solids are brought into contact while they are being transferred. All, or nearly all, of the divided solids are thus heated directly on coming into contact with the heater means, but without remaining stationary against the heating surface. The divided solids are thus heated in uniform manner and, in practice, the problems of crust formation are eliminated. In addition, the structure of the apparatus is simple, so its manufacturing cost remains low.
In a first embodiment, the helical portion of the transfer member comprises a helically-wound flat strip.
In a second embodiment, the helical portion is constituted by a helix wound around a shaft, the shaft then being made of an insulating material or being formed, at least on its outer surface, of an electrically conductive material.
Preferably, the electrically conductive material is a metal such as stainless steel.
Advantageously, the tubular case is constituted by an electrically insulating material. In this way, the risks of short circuits between the transfer element and the tubular case are eliminated, as are the risks of personnel being electrocuted by coming into contact with the tubular case. The safety of the apparatus is thus reinforced.
The tubular case is preferably covered at least in part by an internal friction lining of insulating material against which the helical portion bears. In this way, the friction lining supports the helical portion in such a manner as to prevent the helical portion from sagging while transferring solids. In addition, the friction lining isolates the helical portion electrically from the tubular case, which case can then be made of a conductive material. The risks of short circuits are then limited.
Also advantageously, the apparatus includes a coupling joint of electrically insulating material connecting the transfer member to the outlet shaft of the motor. The motor is thus insulated from the transfer member. The safety of the apparatus is thus further improved.